242 ZOOPHYTA ASCIDIOIDA. 



The polype then is expanded and wishes to sink within its cell. 

 To answer this wish two distinct sets of muscles are provided, 

 the one acting upon the animal, and the other upon the flexible 

 part of the cell, or, in other words, on that portion of the inner 

 tunic which has been carried outwards in the polype's egress. 

 The former set originate from the bottom of the cell («, 8,) and 

 from the sides near the bottom (ar, 9) ; whence trending upwards, 

 they are inserted into the stomach, and into the pharynx and 

 tentacular ring. It is obvious from this position, that their con- 

 traction will draw the whole body down in the cell, where it lies 

 doubled up in the form of a letter S, that all its parts may be 

 brought within the compass of its house. The other set of 

 muscles for the retraction of the flexible portion of the cell, or 

 operculum, have their origin from the inner surface and near the 

 top of the fixed immotive part, and are inserted into the piece 

 on which they act ; (Fig. b, 2, 3,) so that when the first set have 

 partially done their duty, the second follow up the process by 

 pulling after the sinking body the flexible membrane, inverting 

 it at the same time as we undo a stocking from the leg ; and as 

 the last result, closing the aperture or fixing the operculum down 

 upon it. 



To replace the polype in its external relations a very diffe- 

 rent mechanism is brought into play. The inner tunic of the 

 cell, or that which immediately invests the body and encloses the 

 viscera as in a sac, appears to be susceptible of changes in its 

 capacity from the action of some muscular fibres that run in a 

 transverse direction through it. When the polype lies perdue, 

 the sac is distended and the muscles relaxed, but on their con- 

 traction the capacity of the sac is reduced ; and the body, 

 pressed upon on all sides, must of necessity rise upwards in that 

 direction in which it is most free to move. At the same time 

 the little creature appears to have the power of straightening the 

 alimentary canal ; and by its pressure against the bottom of the 

 cell, the extrusion of the body is aided and completed.* 



* It would appear as if muscular fibre were here reduced to its simplest con- 

 dition. The filaments are totally disconnected, and are arranged the one above 

 the other in a single series. They pass straight and parallel from their origin 

 to their insertion, and have a uniform diameter throughout their course, except 

 that each filament generally presents a small knot upon its centre, which is most 



